Disease in plants results from biotic and abiotic causes. Biotic causes include, but are not limited to, fungi and nematodes. A host of cellular processes enables plants to defend themselves from disease caused by pathogenic agents. These processes apparently form an integrated set of resistance mechanisms that is activated by initial infection and then limits further spread of the invading pathogenic organism.
Subsequent to recognition of a plant pathogen, plants can activate an array of biochemical responses. Generally, the plant responds by inducing several local responses in the cells immediately surrounding the infection site. The most common resistance response observed in both nonhost and race-specific interactions is termed the “hypersensitive response” (HR). In the hypersensitive response, cells contacted by the pathogen, and often neighboring cells, rapidly collapse and dry in a necrotic fleck. Other responses include the deposition of callose, the physical thickening of cell walls by lignification, and the synthesis of various antibiotic small molecules and proteins. Genetic factors in both the host and the pathogen determine the specificity of these local responses, which can be very effective in limiting the spread of infection.
Incidence of plant diseases has traditionally been controlled by agronomic practices that include crop rotation, the use of agrochemicals, and conventional breeding techniques. The use of chemicals to control plant pathogens, however, increases costs to farmers and causes harmful effects on the ecosystem. Consumers and government regulators alike are becoming increasingly concerned with the environmental hazards associated with the production and use of synthetic agrochemicals for protecting plants from pathogens. Because of such concerns, regulators have banned or limited the use of some of the most hazardous chemicals. The incidence of fungal diseases has been controlled to some extent by breeding resistant crops. Traditional breeding methods, however, are time-consuming and require continuous effort to maintain disease resistance as pathogens evolve. See, for example, Grover and Gowthaman (2003) Curr. Sci. 84:330-340. Thus, there is a significant need for novel alternatives for the control of plant pathogens that possess a lower risk of pollution and environmental hazards than is characteristic of traditional agrochemical-based methods and that are less cumbersome than conventional breeding techniques.
Recently, agricultural scientists have developed crop plants with enhanced pathogen resistance by genetically engineering plants to express antipathogenic proteins. For example, potatoes and tobacco plants genetically engineered to produce an antifungal endochitinase protein were shown to exhibit increased resistance to foliar and soil-borne fungal pathogens. See Lorito et al. (1998) Proc. Natl. Acad. Sci. 95:7860-7865. Moreover, transgenic barley that is resistant to the stem rust fungus has also been developed. See Horvath et al. (2003) Proc. Natl. Acad. Sci. 100:364-369. A continuing effort to identify antipathogenic agents and to genetically engineer disease-resistant plants is underway.
Nematode infection is a significant problem in the farming of many agriculturally significant crops. For example, soybean cyst nematode (Heterodera glycines) is a widespread pathogen that is believed to be responsible for yield losses in soybeans, estimated to be in excess of $1 billion per year in North America. Such damage is the result of the stunting of the soybean plant caused by the cyst nematode. The stunted plants have smaller root systems, show symptoms of mineral deficiencies in their leaves, and wilt easily. Other pathogenic nematodes of significance to agriculture include the potato cyst nematodes Globodera rostochiensis and Globodera pallida, which are key pathogens of the potato, the cotton cyst nematode, Meloidogyne incognita, and the beet cyst nematode Heterodera schachtii, which is a major problem for sugar beet growers in Europe and the United States.
The primary method of controlling nematodes has been through the application of highly toxic chemical compounds. The widespread use of chemical compounds poses many problems with regard to the environment because of the non-selectivity of the compounds and the development of insect resistance to the chemicals. Nematicides such as Aldicarb and its breakdown products are known to be highly toxic to mammals. As a result, government restrictions have been imposed on the use of these chemicals. The most widely used nematicide, methyl bromide, is scheduled to be soon retired from use, and at present, there is no promising candidate to replace this treatment. Thus, there is a great need for effective, non-chemical methods and compositions for nematode control.
Various approaches to pathogen control have been tried including the use of biological organisms which are typically “natural predators” of the species sought to be controlled. Such predators may include other insects, fungi, and bacteria such as Bacillus thuringiensis. Alternatively, large colonies of insect pests have been raised in captivity, sterilized and released into the environment in the hope that mating between the sterilized insects and fecund wild insects will decrease the insect population. While these approaches have had some success, they entail considerable expense and present several major difficulties. For example, it is difficult both to apply biological organisms to large areas and to cause such living organisms to remain in the treated area or on the treated plant species for an extended time. Predator insects can migrate and fungi or bacteria can be washed off of a plant or removed from a treated area by rain. Consequently, while the use of such biological controls has desirable characteristics and has met with some success, in practice these methods have not achieved the goal of controlling pathogen damage to crops.
Advances in biotechnology have presented new opportunities for pathogen control through genetic engineering. In particular, advances in plant genetics coupled with the identification of naturally-occurring plant defensive compounds or agents offer the opportunity to create transgenic crop plants capable of producing such defensive agents and thereby protect the plants against disease.
One promising method for nematode control is the production of transgenic plants that are resistant to nematode infection and reproduction. For example, with the use of nematode-inducible promoters, plants can be genetically altered to express nematicidal proteins in response to exposure to nematodes. See, for example, U.S. Pat. No. 6,252,138, herein incorporated by reference. Similarly, with the use of proteins having a deleterious effect on nematodes, plants can be genetically altered to express such deleterious proteins in response to nematode attack.
Thus, in light of the significant impact of plant pathogens, particularly fungal and nematode pathogens, on the yield and quality of crops, new methods for protecting plants from such pathogens are needed.